The disclosure relates generally to processes for recovering paraxylene from at least two feed streams containing xylene isomers, particularly the integration of recovery of paraxylene from a paraxylene-lean feedstream and a paraxylene-rich feedstream. More specifically, the disclosure relates to processes incorporating the use of a paraxylene recovery zone comprising at least one crystallization zone, and optionally at least one reslurry zone.
Paraxylene (p-Xylene or pX), in purified form, is useful for making terephthalic acid. Paraxylene is generally obtained by separation from Mixed Xylenes. In the industry, Mixed Xylenes refer to a narrow boiling distillation heart cut of the C8 aromatics comprising the three xylene isomers orthoxylene (oX), metaxylene (mX) and paraxylene (pX), as well as ethylbenzene (EB). Mixed Xylenes may also contain non-aromatic compounds with boiling points close to the xylenes. These mainly comprise C9 paraffins and naphthenes. Mixed Xylenes generally also contain low levels of toluene and C9 and higher aromatics present due to their imperfect separation in the distillation towers used to produce the Mixed Xylenes heart cut. ASTM publishes a Standard Specification for Xylenes for Paraxylene Feedstock (ASTM D 5211-07 or subsequent versions); however, specifications that deviate from this are often set between Mixed Xylenes purchasers and suppliers.
The proportions of xylene isomers and ethylbenzene present in Mixed Xylenes will vary depending upon the source of the Mixed Xylenes. Mixed Xylenes are typically a narrow boiling distillation cut obtained from a reformate of the refinery catalytic reformer unit or another unit used to produced Mixed Xylenes, such as a non-selective toluene disproportionation (TDP) unit, a selective toluene disproportionation (STDP) unit, a non-selective or selective toluene alkylation unit, a toluene/aromatic C9-plus transalkylation (TA) unit or an aromatic C9-plus transalkylation unit. Toluene alkylation units react toluene with methanol to produce xylene isomers and water, although other side reactions can take place, such as the conversion of part of the methanol to alkanes, olefins and other alkylation products. Processes for producing Mixed Xylenes may be “selective” or “non-selective” processes. A non-selective process generally produces xylene isomers with a near equilibrium distribution of the xylene isomers (paraxylene:metaxylene:orthoxylene) of approximately 1:2:1, i.e. the ratio of paraxylene to total xylene isomers is approximately 0.25. In contrast, a process that is selective for paraxylene yields a paraxylene concentration above the theoretical equilibrium, i.e. the ratio of paraxylene to total xylene isomers is greater than 0.25. Thus, xylenes produced in catalytic reformers, conventional TDP units, conventional (non-selective) toluene alkylation units, and toluene/aromatic C9-plus or aromatic C9-plus transalkylation units generally contain xylene isomers with a near equilibrium distribution (i.e. with a ratio of paraxylene to total xylene isomers of approximately 0.25). In comparison, the Mixed Xylenes distillation cut from an STDP unit or a selective toluene alkylation unit can have a paraxylene to total xylene isomer ratio of greater than 0.7, more typically above 0.8, and often above 0.9.
Thus, a paraxylene producer may purchase Mixed Xylenes as feed to a paraxylene unit, or they may purchase or produce other sources of xylene isomers, all having widely different proportions of paraxylene therein. Where the proportion of paraxylene in the Mixed Xylenes is relatively low, a first paraxylene separation stage may be carried out and the remaining paraxylene-depleted stream may be further processed to produce additional paraxylene. Such a process may be carried out in a paraxylene unit, which is typically comprised of three sections: a paraxylene recovery section, an isomerization section, and a fractionation section. The purpose of the paraxylene recovery section is to generate a paraxylene product stream and a paraxylene lean stream, known as reject or raffinate. The paraxylene lean stream is directed to the isomerization section that comprises a reactor and a catalyst used to isomerise the xylenes in the reject stream to a near equilibrium distribution. The catalyst should also convert any ethylbenzene present in the mixture to either xylenes or by-products that can readily be separated in the fractionation section, to prevent its build up in a recycle loop generated within the paraxylene unit. Any non-aromatics present should also be converted, typically by cracking to smaller hydrocarbons to prevent their build up.
Xylene isomerization catalysts are typically categorized by the way they convert ethylbenzene. For example, ethylbenzene isomerization-type catalysts (also known as naphthene pool catalysts) have the ability to convert a portion of the ethylbenzene to xylene isomers via C8 naphthene intermediates. Ethylbenzene dealkylation-type catalysts convert ethylbenzene primarily via reaction with hydrogen to form benzene and ethane. Ethylbenzene transalkylation-type catalysts convert ethylbenzene primarily by the transfer of the ethyl group to another ethyl benzene or to a xylene. All of these catalysts produce by-products from the ethylbenzene conversion reactions and/or side reactions that must be separated in the fractionation section. These by-products include benzene, toluene, and C9-plus aromatics. Benzene is a valuable by-product, and is generally recovered in high purity by additional fractionation equipment or by extraction or extractive distillation.
Two popular methods for recovering paraxylene in the paraxylene recovery section are crystallization and selective adsorption. Selective adsorption processes include the UOP Parex process described in R A Meyers (editor) Handbook of Petroleum Refining Processes, Third Edition (2004) and the Axens Eluxyl process described in G Ash, et al, Oil and Gas Technology, 49 (5), 541-549 (2004).
Paraxylene crystallization recovery sections generally contain several stages in order to achieve final product purity and to improve efficiency. Examples include a two stage paraxylene crystallization recovery section comprising two crystallization stages, a three stage paraxylene crystallization recovery section comprising three crystallization stages, a single reslurry paraxylene crystallization recovery section comprising two crystallization stages and one reslurry stage, and a double reslurry paraxylene crystallization recovery section comprising one crystallization stage and two reslurry stages.
In one known process, a single temperature crystallization product stage is used for producing paraxylene from a feed having an above equilibrium paraxylene concentration, such as from a toluene disproportionation process. Scavenger stages are also used to raise the paraxylene recovery rate. The process uses a single temperature production stage comprising one or more crystallizer vessels in parallel, i.e. the paraxylene-rich stream is fed to the crystallization stage and not to a reslurry stage.
A further known process uses crystallization technology to purify paraxylene simultaneously of large concentrations of C8 aromatics and also small concentrations of oxygenated species. This process comprises a first stage in which a paraxylene feed is cooled, crystallized and separated at a very cold temperature for maximum recovery, following which the crystals are melted and recrystallized and separated at a warmer temperature.
U.S. Pat. No. 6,565,653 relates to a process to produce high purity paraxylene from a feed comprising at least 55 to 60 wt % paraxylene, wherein a first portion of the high purity paraxylene is obtained in a first crystallization step at about 10° F. to about 55° F. without the need for further reslurry and crystallization, and wherein another portion of the high purity paraxylene product is obtained following a reslurry step, which warms crystalline paraxylene obtained from subsequent lower temperature crystallizations to yield a slurry at a temperature of about 10° F. to about 55° F., without the need for further refrigeration. The disclosed process includes crystallization stages and reslurry stages, but the first step of the process is crystallizing the paraxylene-rich feedstream in a first crystallizer.
U.S. Pat. No. 7,405,340 relates to a process for recovering paraxylene that comprises cooling the hydrocarbon feedstock in at least one refrigerated crystallization stage that is indirectly refrigerated by evaporating at least a portion of a substantially liquid stream comprising ammonia. The process is said to be carried out using a crystallization paraxylene recovery section for recovering paraxylene from paraxylene-rich STDP xylenes with each stage comprised of crystallizers, and where heat can be removed from the crystallizers via indirect contact with an ammonia refrigerant. The ammonia refrigeration cycle is an ammonia absorption refrigeration cycle.
U.S. Pat. No. 2010/0041936 relates to a process for separating solids from a solid-liquid slurry, such as paraxylene from a Mixed Xylene slurry, incorporating a crystallization stage and one or more reslurry stages. In a particular embodiment, the process is said to include two reslurry stages, and the ratio of paraxylene to total xylene isomer, in the product of each stage is said to generally increase throughout the process.
In separating paraxylene from a C8 aromatic hydrocarbon feed, crystallization is often preferred over adsorption and distillation because crystallization does not require a costly adsorbent (as in adsorption processes), and because xylene isomers and ethylbenzene have undesirably similar boiling points (making distillation difficult) but dramatically different melting points. Pure paraxylene freezes at 56° F. (13° C.), pure metaxylene freezes at −54° F. (−48° C.), pure orthoxylene freezes at −13° F. (−25° C.) and pure ethylbenzene freezes at −139° F. (−95° C.). Where paraxylene is present in such mixed feedstreams in low concentrations, very low temperatures are generally required to effectively recover the paraxylene from the feedstreams by crystallization.
There remains a need to find processes that seek to optimize the recovery of paraxylene from mixed feedstreams thereof.